Optical storage media are media in which data are stored in an optically readable manner, for example by means of a pickup comprising a laser for illuminating the optical storage medium and a photo-detector for detecting the reflected light of the laser beam when reading the data. In the meanwhile a large variety of optical storage media are available, which are operated with different laser wavelength, and which have different sizes for providing storage capacities from below one Gigabyte up to 50 Gigabyte (GB). The formats include read-only formats (ROM) such as Audio CD and Video DVD, write-once optical media as well as rewritable formats. Digital data are stored on these media along tracks in one or more layers of the media.
The storage medium with the highest data capacity is at present the Blu-Ray disc (BD), which allows to store 50 GB on a dual layer disc. Available formats are at present for example read-only BD-ROM, re-writable BD-RE and write once BD-R discs. For reading and writing of a Blu-Ray disc an optical pickup with a laser wavelength of 405 nm is used.
On the Blu-Ray disc a track pitch of 320 nm and a mark length from 2 T to 8 T, maximum 9 T, is used, where T is the channel bit length, which corresponds with a length of 69-80 nm. Further information about the Blu-Ray disc system is available for example from the Blu-Ray group via Internet: www.blu-raydisc.com.
New optical storage media with a super-resolution near-field structure (Super-RENS) offer the possibility to increase the data density of the optical storage medium by a factor of three to four in one dimension in comparison with the Blu-Ray disc. This is possible by using a so-called Super-RENS structure or layer, which is placed above the data layer of the optical storage medium, and which significantly reduces the effective size of a light spot used for reading from or writing to the optical storage medium. The super-resolution layer is also called a mask layer because it is arranged above the data layer and by using specific materials only the high intensity center part of a laser beam can penetrate the mask layer.
The Super-RENS effect allows to record and read data stored in marks of an optical disc, which have a size below the resolution limit of a laser beam used for reading or writing the data on the disc. As known, the diffraction limit of the resolution of a laser beam is about lambda/(2*NA) according to Abbe, where lambda is the wavelength and NA the numerical aperture of the objective lens of the optical pickup.
A Super-RENS optical disc comprising a super-resolution near-field structure formed of a metal oxide or a polymer compound for recording of data and a phase change layer formed of a GeSbTe or a AgInSbTe based structure for reproducing of data is known from WO 2005/081242 and US 2004/0257968. Further examples of super-resolution optical media are described in WO 2004/032123 and by Tominaga et al., Appl. Phys. Lett. Vol. 73, No. 15, 12 Oct. 1998.
The super RENS effect allows to increase the resolution of the optical pickup for reading of the marks on an optical disc in track direction, but does not allow to reduce the track pitch.